Guided Wave Radar (GWR) level transmitters are widely used for process level measurement and process control in a variety of applications including chemical, petrochemical and medical, as well as in custody transfer, marine and transportation. GWR level transmitters work on the principle of time domain reflectometry (TDR), where the time elapsed between sending a microwave pulse along a metal probe which is guiding it, and receiving its “echo” reflected from the surface and/or interface of interest is measured and analyzed. The half trip time multiplied by the light velocity provides the level and/or interface depending on application requirement.
For GWR radar operation, a short electromagnetic (E-M) pulse with a duration from much less than 1 ns (100 ps) to about 10 ns is generated in an electronic block (transceiver) which is propagated with light velocity along a 50-100Ω coaxial cable to the tank input. Such a 50-100Ω coaxial cable acts as a transmission line allowing mainly electromagnetic waves of transversal electric magnetic mode (TEM) mode (Ez=0, Hz=0) to propagate through it, while the other electromagnetic wave modes propagating therein are negligible due to the specific geometrical construction of the cable (inner conductor diameter, outer conductor diameter and type of dielectric used in between them). The coaxial line is generally connected to a 50-100Ω coupling device (“feed-through”) acting as a coaxial transmission line. Other GWR systems for level measurement replace the coaxial cable with a transmission line on a printed circuit board assembly (PCBA) terminated on a coaxial connector which is—coupled to the process connector.
From this “feed-through” device, the electromagnetic signal is further guided by the probe and enters either directly in the opening of the tank (threaded process connection), or in other cases, a guided wave travels first through a “tank interface” referred to as a nozzle, and then enters the tank. The nozzle is a small cylinder having a mounting flange at one end, while at the other end it is welded to the tank.
The nozzle's diameter is in general, larger than its height, but not always. Once entering the tank, the electromagnetic (E-M) pulse is guided by the probe in the free space area of the tank, where the free space impedance is about 377Ω. Independent of their geometries, nozzles in conjunction with a central electrical conductor more or less functions as a coaxial transmission line of higher impedance with respect to the 50-100Ω impedance of coaxial cable or feed-through, but smaller than the free-space impedance, of 377Ω.
According to this analogy, the impedance of the nozzle can be calculated like in the case of coaxial transmission lines, by the formula Zh=(60/∈r1/2)*ln(D/d), where “D” is the diameter of the outer conductor (i.e., the nozzle wall), “d” is the diameter of the single conductor probe going through it, while ∈r is the dielectric constant of the material present between the outer conductor and inner conductor, (in this case, air). However, nozzles of diameters larger than a certain value are not always similar to the “standard” coaxial line, as they allow the propagation of E-M waves of frequencies higher than a cutoff frequency approximated by the formula fc=190.85/[D+d)*∈r1/2], where “D” and “d” are in mm, the nozzles also support higher order E-M modes including transversal electric (TE11) mode (Ez=0, Hz≠0). Such a TE11 mode will propagate with a different phase velocity and will interfere with the TEM mode thus creating parasitic reflections, called “ringing”, which will decrease the accuracy of level measurement closer to the top of the tank, and, will thus decrease the maximum level range. Such a ‘ringing” effect will be visible even for nozzle diameters (D) equal to 4″ (4 inches=10.16 cm) for interrogation pulses of duration shorter than 0.25 ns, for which, a larger portion of the frequency bandwidth will be higher than the above cutoff frequency. The impedance mismatch between the “feed-through” device and the nozzle entrance causes the first major reflection of the E-M wave, but, the time at which this first reflection occurs can be useful as a time reference for pulse runtime.
The impedance mismatch between the output of the nozzle and the free-space impedance of the probe, located deeper inside the tank also generates parasitic E-M reflections. These parasitic reflections at the input and output of the nozzle reduce the remaining energy of the E-M wave to be used for level measurement of the product, and thus reduce the maximum range of the level measurement, taking also into account the attenuation along the probe. In addition, these parasitic reflected waves also reduce the ability to accurately measure the level of the product near the top of the tank. From this reason, GWR level transmitters generally define an upper dead-zone, inside which the product level cannot be measured. The higher the impedance mismatches, the larger the dead-zones and more accuracy is reduced in level measurement near the top of the tank. Different approaches have been used in an attempt to minimize these parasitic reflections.
One approach for reducing the reflections from the end of the tank nozzle utilizes a tapered thickness dielectric coating layer on the probe starting below the nozzle. The tapered dielectric coating introduces a somewhat smoother impedance transition from nozzle impedance to the free space impedance, of about 377Ω. Although there is some reduction of feed-through echo from this approach, the ringing effect due to larger nozzle diameter and/or lower duration of the interrogation pulse is still not solved. Moreover, this known approach is dependent on the particular nozzle geometry, with amplitude of echo coming from the end of feed-through/nozzle entrance which is increasing with nozzle diameter increase.